Semiconductor devices may be classified into I/O devices and core devices in accordance with functions thereof, in which both the I/O devices and core devices generally include horizontal transistors. Since higher voltage is applied to the I/O device compared to the core device, a length of a gate and a thickness of a gate dielectric layer of the horizontal transistor of the I/O device should be much greater than those of the core device, which results in great occupied area and poor integration density.
In order to improve integration density, vertical transistors with the same gate length in the I/O devices and the core devices may be applied. However, gate length and channel length of the vertical transistor are much less than that of the horizontal transistor, such that drain induced barrier lowering (DIBL) and hot carrier injection (HCI) phenomena may occur in the vertical transistor of the I/O device when high voltage is applied, and thus to increase leakage current and significantly reduce reliability. Accordingly, improvements in the vertical transistor of the I/O device continue to be sought.